variable growth factor.js

var growthFactor = function(totalTweets) {

  var lowerBoundTweets = 100;
  var lowerBoundGrowth = 1;

  var upperBoundTweets = 200;
  var upperBoundGrowth = 1.5;

  var dx = upperBoundTweets - lowerBoundTweets;
  var dy = upperBoundGrowth - lowerBoundGrowth;

  var growth = (totalTweets - lowerBoundTweets) * (dy / dx) + lowerBoundGrowth;

  if (growth < lowerBoundGrowth) return lowerBoundGrowth;
  if (growth > upperBoundGrowth) return upperBoundGrowth;

  return growth;

}